Characteristics of Good Building Stones

1. Appearance. The stones should be of uniform colour and from architectural point of view, these should match with the surroundings. Lighter colours are preferred to darker ones as the latter are less durable. The property of the appearance of stones is extremely important for the face work of buildings. The good stones should also be able to receive good polish.

2. Strength. The stones used in the construction of buildings are generally subjected to compressive stress. These should therefore, be able to withstand the compression without getting crushed due to the load of the structure. Though all stones used for the construction of ordinary buildings, possess workable strength in compression but the strength of stones used for the construction of heavy structures should always be tested before use. Closed grained and uniform

textured stones are generally good in compressive strength.

3. Structure. A good stone when broken in a direction other than that of cleavage .(if any) should not give a dull appearance. These should be either closed grained or crystalline and should show uniformity of texture. These should also be free from cavities and cracks. If the stones are obtained from sedimentary rocks, their stratification should not be visible to naked eye.

4. Hardness. A good stone when used in floors, pavements and aprons of bridges,should be able to resist the abrasive forces caused due to wear and friction. Hardness of stones can be tested by Mohr's scale of hardness in the laboratory whereas in field, it can be tested by knife scratching. Hard stones do not show any mark of scratching.

5. Toughness. Good stones should also be tough enough to withstand stresses developed due to vibrations of machineries and moving loads over them. Stones used in the construction of roads should be hard as well as tough.

6. Heaviness. The stones used for the construction of dams, weirs, barrages, docks and harbours, should be of heavier varieties. In case of dams and roof coverings, lighter varieties of stones are preferred to. The specific gravity of good building stones should lie between 2.4 to 2.8.

7. Durability. Good stones should resist the action of the atmosphere such as wind, rain and temperature. The effect of atmospheric conditions on stones is generally known as weathering. Durability of stones largely depends upon their chemical composition and physical structure. A durable stone should have a compact and crystalline structure, free from pores.

8. Porosity and absorption. A good stone should not be porous. It should not absorb water when immersed. Porous stones are unsuitable for the construction work as rain water falling on their exposed surface get driven in the pores of stones by the prevailing winds. The rain water generally becomes acidic due to atmospheric acidic gases and this acidic water reacts with the constituents of

stones causing them to crumble. In higher region water in the pores when freezes, disintegrates the stones because of increased volume after freezing. Porous stones should be used in the construction of structures with care and at places which are not exposed to frost, rain or moisture.

9. Resistance to fire. Stones when exposed to fire should be able to resist temperature. To ensure this, stones should be free from minerals which are likely to decompose on heating such as calcium carbonate and iron oxide. Stones should not be composed of minerals having different coefficients of thermal expansion. Quartz expands at low temperature and argillaceous variety of

stones resist high temperature.

10. Dressing. The art of converting a natural stone into a definite shape, is known as dressing. Stones should therefore possess good dressing properties for carving and structural work in building constructions. Marble is a good example. It may be noted that a stone possessing good dressing properties on the other hand, is weak in strength and durability, and also its hardness is low.

11. Seasoning. A good stone should be free from quarry sap. To ensure this, the stones after quarrying and dressing should be left for a period of 6 to 12 months for proper seasoning, before using it in the construction work.

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